1. Field of the Invention
The present invention relates to a chemical vapor deposition apparatus and a method of manufacturing a semiconductor device, and more specifically, to a chemical vapor deposition (CVD) apparatus for forming a dielectric film applied to a semiconductor memory device and a method of manufacturing a semiconductor device using the chemical vapor deposition apparatus.
2. Description of the Background Art
In recent years, increasingly higher degree of integration is being achieved in semiconductor memories and semiconductor devices at a great speed. For instance, a dynamic random access memory (DRAM) has undergone a rapid increase in the bit number: it has quadrupled in three years. The aims are to achieve higher degree of integration of the device, lower power consumption, lower cost, and so on. A capacitor, which is a component of a DRAM, however, is required to have a constant capacitance regardless of the improved degree of integration of the device.
One way of ensuring the capacitance of a capacitor is to create a thin capacitor insulating film. With the silicon oxide film (SiO.sub.2) that has been used until now, however, there are limits as to how thin the film could be formed.
Consequently, as another way of ensuring the capacitance of a capacitor, the material for the capacitor insulating film has been changed. In other words, a thin film formed of a material having a high dielectric constant came to be utilized as the capacitor insulating film.
Oxide-type dielectric films as examples of high dielectric constant materials including, for instance, tantalum oxide, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), strontium titanate (ST), barium titanate (BT), barium strontium titanate ([(Ba, Sr)TiO.sub.3 ] hereinafter referred to as "BST"), and the like are being considered.
In order to form such oxide-type dielectric film as a thin film on a capacitor electrode of a DRAM having steps, it is advantageous to employ the CVD method which provides favorable coating onto a surface having a complex shape. In CVD method, a liquid source is used as a source for the thin film having a high dielectric constant. The liquid source is prepared by dissolving an organometallic complex containing a certain metal in an organic solvent. The liquid source is vaporized, and the resultant vapor is blown against the substrate to form a thin film having a high dielectric constant.
A significant problem has been, however, that a liquid source having a stable and good vaporization characteristic does not exist. This is mainly due to the poor vaporization characteristic, upon heating, of the compound of a metal and .beta.-diketon-type dipivaloyl methane (DPM) frequently used as an organometallic complex.
Under these circumstances, the present inventors proposed a CVD source having a greatly improved vaporization characteristic by utilizing a liquid source prepared by dissolving a conventional organometallic complex in an organic solvent called tetrahydrofuran (THF: C.sub.4 H.sub.3 O) (Japanese Patent Laying-Open No. 7-268634).
It was discovered, however, that a dielectric film having a good quality such as a good electrical property could not always be consistently formed when the film was formed with this CVD source using a conventional CVD apparatus for liquid source. Thus, the present inventors proposed in Japanese Patent Laying-Open No. 8-186103 a CVD apparatus for liquid source which allows adequate vaporization of the liquid source and which supplies the vapor stably to the reaction chamber.
Now, the CVD apparatus for liquid source disclosed in the above Japanese Patent Laying-Open No. 8-186103 will be described with reference to the drawing. In FIG. 16, the CVD apparatus for liquid source is provided with liquid source vessels 164 to 167, liquid source flow rate control systems 160 to 163, a vaporizer 156, and a reaction chamber 153. Liquid source vessels 164 to 167 each store a liquid CVD source prepared by dissolving an organic complex containing a prescribed metal in an organic solvent.
To each of the liquid source vessels 164 to 167 a pressure tube 169 is connected, and through pressure tube 169 an inert gas such as nitrogen is fed into each of the liquid source vessels 164 to 167. Consequently, the pressure inside each of the liquid source vessels 164 to 167 rises, causing the liquid CVD sources to be supplied to vaporizer 156. Here, the flow rates are respectively controlled by liquid source flow rate control systems 160 to 163. In addition, a carrier gas such as nitrogen is introduced from a carrier gas feed inlet 168 in order to send the liquid CVD source via a connecting tube 158 to vaporizer 156. Here, the flow rate is controlled by a carrier gas flow rate control system 159.
Liquid CVD source, having reached vaporizer 156, is vaporized therein, and the resultant vapor flows through a source gas conveying tube 155 to a mixer portion 170. Conveying tube heaters 157 are provided around source gas conveying tube 155 to prevent the CVD source gas from turning back into liquid. The CVD source gas and oxygen supplied from an oxidizing agent feed line 154 are mixed in mixer portion 170. The CVD source gas mixed with oxygen is introduced into reaction chamber 153 via a source gas inlet 171, and thereafter, a thin film is formed on a substrate 151.
When forming a BST film as the thin film, liquid sources prepared by respectively dissolving in an organic solvent the organometallic complexes respectively containing barium (Bi), strontium (Sr), and titanium (Ti) were used. Oxygen ambient was provided inside reaction chamber 153, and the pressure was set between 1 and 10 Torr. The temperature of a substrate heater 152 was set to be in the range of 400.degree. C. to 600.degree. C. The flow rates of the liquid sources and the film deposition time were controlled such that the value of the BST film composition ratio (Ba+Sr)/Ti was 1.0. In this case, the film deposition rate was 3 nm/min.
As described above, liquid sources prepared by dissolving the DPM-type organometallic compounds in an organic solvent were used as the CVD sources. The source gas vaporized in vaporizer 156 is introduced into reaction chamber 153 via source gas inlet 171. At this time, a substantially steady flow of source gas from source gas inlet 171 directed to exhaust outlet 172 is formed in reaction chamber 153.
As a result, there was a problem of uneven distribution within the substrate surface regarding the thickness and the composition ratio of the BST film formed on substrate 151. More specifically, the film thickness tended to be relatively thick on the side where exhaust outlet 172 was provided. As regards the film composition ratio, the film tended to contain more titanium (Ti) than barium (Ba) or strontium (Sr) nearer to the exhaust outlet.
Moreover, the attempt to rotate the substrate to eliminate the unevenness within the substrate surface caused the problem of particle generation accompanied by the rotation.
Further, in the above-described CVD apparatus for liquid source, the source gas introduced from the inlet diffused inside reaction chamber 153 so that the vapor was not effectively brought onto substrate 151, which led to the problem of a low "use efficiency" or the ratio of the amount of the source gas contributing to film growth to the amount of the source gas supplied being low.
In addition, in a conventional CVD apparatus for liquid source, some of the heat from substrate heater 152 was absorbed by a wall of reaction chamber 153 having a low temperature so that a portion having a relatively low temperature was created within reaction chamber 153, causing the source gas introduced into reaction chamber 153 to condense in that portion. As a result, the condensed source gas was attached onto the substrate 151 as particles of foreign substance.
This, moreover, lead to another problem that the source gas could not be effectively brought onto substrate 151 due to the condensing of the source gas.